1. Field of the Invention
The present invention relates to a spread spectrum communication system divided into a plurality of cells, such as a CDMA (Code Division Multiple Access) system.
2. Description of the Prior Art
A conventional example of a power controller for use in the spread spectrum CDMA radio communication system varies a process gain in accordance with a location in a cell where a mobile station exists as disclosed in JPA 9-284212.
Process gain PG is derived from spread rate C (chip/sec)=1/Tc which is the reciprocal of 1 chip time Tc of Pseudo Noise series and data transmission rate D (bps), and is represented as PG=C/D=TcD (chip/bit). Increasing spread rate C of direct spread reduces a power spectral density and expands a spread bandwidth. If data transmission rate D is constant, process gain PG increases characteristically when spread rate C increases.
Therefore, if spread rate C is increased while sustaining a constant transmission rate of a mobile station that is located close to a certain base station relative to a mobile station that is located remotely, the process gain for the closely located mobile station increases and the power spectral density of the signal received at the base station from the closely located mobile station decreases so that the power spectral densities received at the base station from the respective mobile stations become substantially equal level.
On the other hand, if the data transmission rate is decreased while sustaining a constant chip rate of the mobile station that is located remotely from the base station relative to the mobile station that is located closely, the process gain increases and an S/N ratio (Signal-to-Noise Ratio) increases.
Thus, such distance problem existing among the mobile stations located close to the station and those located remotely from the base station can be solved, and in addition, interference to other stations can be suppressed.
FIG. 1 is a block diagram showing an arrangement of the above spread spectrum communication system. FIG. 1 shows a transmitter/receiver section, which includes SS (Spread Spectrum) transmitter 1 and SS receiver 2, provided in a base station and each mobile station. In SS transmitter 1, a signal containing audio data, information data and image data is primarily modulated by a data clock supplied from data clock generator 3 at information transmitter 4 so as to be another signal having a predetermined data transmission rate D. Thus generated signal is fed to next spreading modulator 5.
PN clock is fed to PN generator 6 from PN clock generator 7. PN generator 6 generates PN signal 8 with a predetermined spread rate C accordingly. The PN signal is fed to spreading modulator 5.
The signal from information transmitter 4 is directly spread by PN signal 8 at spreading modulator 5. The directly spread signal (hereinafter, referred to as an SS signal) 9 is converted into the SS signal having a radio frequency by frequency converter 10, then amplified by power amplifier 11, and transmitted from antenna 12.
On the other hand, in SS receiver 2, the SS signal received at antenna 13 is amplified by amplifier 14, and then is converted into the SS signal having an intermediate frequency or baseband frequency. It is subsequently synchronized correlatively at correlation unit 16, and then demodulated into the original signal by information demodulator 18.
In the above configuration, data transmission rate D is controlled by varying a clock speed of data clock generator 3, and spread rate C is controlled by varying a clock speed of PN clock generator 7. Thus, the process gain can be controlled by controlling these clock speeds.
The above prior art, however, includes a disadvantage that increasing spread rate C of the closer mobile station widen a frequency bandwidth and reduces frequency usage efficiency. There is also a disadvantage that a circuit scale of controller for increasing spread rate C becomes large, and especially it becomes troublesome for the mobile station.
In order to overcome the aforementioned disadvantages, the present invention has been made and accordingly, has an object to provide a spread spectrum communication system divided into a plurality of cells (for example, the CDMA (Code Division Multiple Access) system) in which communications in one cell does not communications in adjacent cells.
According to an aspect of the present invention, there is provided a spread spectrum communication system comprising a plurality of cells which share common frequencies, wherein each base station managing each cell comprises: a first transmitting means for transmitting a reception load state of the own base station to other base stations managing other cells; a first receiving means for receiving reception load states of the other base stations from the other base stations managing the other cells; a second transmitting means for transmitting a signal having control information interpolated therein to each mobile station within the cell under management of the own base station, the control information being used for controlling a transmission state of the each mobile station; a second receiving means for receiving a signal from the each mobile station within the cell under management of own base station; and a power control means for determining the control information for the each mobile station so as not to interfere with operations of the other base stations managing the other cells with reference to a reception level index of the signal from the each mobile station and the reception load states of the other base stations managing the other cells.
In the spread spectrum communication system, the control information may be set to special information for lowering or cutting off a transmission power of the each mobile station if it is estimated that the transmission power of the each mobile station for gaining a desired value of the reception level index of the signal from the each mobile station interferes with the operations of the other base stations managing the other cells.
In the spread spectrum communication system, the estimation of interference may be executed with reference to the reception load states of the other base stations managing the other cells.
In the spread spectrum communication system, the estimation of interference may be executed in consideration of a relative relationship between a location of the each mobile station and locations of the other base stations managing the other cells, the location of the each mobile station being computed on the basis of the control information and the signal from each mobile station.
In the spread spectrum communication system, the special information may comprise such information as halts a transmission operation of the mobile station.
In the spread spectrum communication system, the special information may comprise such information as lowers the transmission power of the mobile station.
In the spread spectrum communication system, the special information may comprise such information as decreases a transmission rate of the mobile station.
In the spread spectrum communication system, contents of the control information sent to the each mobile station may be decided so as to gain a desired value of the reception level index of the signal from each mobile station if it is not estimated that the transmission power of the each mobile station for gaining the desired value of the reception level index of the signal from the each mobile station interferes with the operations of the the other base stations managing the other cells.
In the spread spectrum communication system, contents of the control information sent to the each mobile station may be decided so as to gain a desired value of the reception level index of the signal from each mobile station if a transmission power of the each mobile station for gaining the desired value of the reception level index of the signal from the each mobile station is less than a certain upper limit value.
In the spread spectrum communication system, the certain upper limit value may be determined in accordance with distances of the other base stations managing the other cells from the own base station.
In the spread spectrum communication system, the first transmitting means may transmit the reception load state of the own base station during the first receiving means is not receiving the reception load states from the other base stations managing the other cells.
In the spread spectrum communication system, the first transmitting means may transmit the reception load state of the own base station in time-division together with the first receiving means of the other base stations managing the other cells.
In the spread spectrum communication system, the power control means may determine the control information of the each mobile station dependently on every area formed by dividing the cell under management of the own base station.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof.